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Description: A hand-out for my daughter's "cryptographic treasure hunt" birthday party. This was written by Dylan McName. The rest of the party is documented here: http://dylanmcnamee.vox.com/library/post/crypt...

A hand-out for my daughter's "cryptographic treasure hunt" birthday party. This was written by Dylan McName. The rest of the party is documented here: http://dylanmcnamee.vox.com/library/post/crypto-treasure-hunt-birthday-party-a-great-success.html

Introduction
So, you’ve got some codes to crack, and you want to know how to get started? You’ve come to the right place. This book will help you gure out what to do in any puzzling situation. The main things to keep in mind are “stay calm,” and “I can gure this out.”

The birthday invitation code is a simple transposition cypher. Here’s the key for that code:

3

Invisible ink
If you have a blank piece of paper that you believe has a message written on it in invisible ink, you’re halfway there. Most people would just throw a blank sheet of paper away, or use it for something else, or recycle it, or turn it into a paper airplane. You secret agents know better though, don’t you? If you have an invisible ink pen & developer, that might be exactly what you need to read the message, but Be Careful! The ink & developer can be reversed, and if you scribble the wrong one all over your paper, you’ll be covering up the secret message! Try small corners or careful patterns to determine if an invisible ink has been used on the paper. Another form of invisible ink is lemon juice. The way you make these messages appear is with a candle. But once again: Be Careful! If the paper catches re, not only could you hurt yourself (or worse), but you’ll also lose the secret in the process.

4

Morse code
This tree should help you decode morse code. When you see (or hear!) a pause, wherever you are in the tree is your letter. So “dee-dee” is the letter “i”, while “dah dee dah” is “k”. Take your time.

Transposition cyphers
A transposition cypher is where each letter in a word is replaced with another symbol/letter/or number. Transposition cyphers are the basis of modern cryptography. The simplest transposition cypher is the obvious A -> 1, B -> 2, C -> 3, and so on. This code would always be easy to break, even if you scrambled up which numbers went with which letters. Why? Because English text (and most other languages) leaves “ ngerprints” - some letters occur very frequently (like “a” and “e”), and others are more rare (“q” and “z”), and some words are very common (like “the”), so if you have enough text to decode, these rules can help you get most of the way there. And even if you don’t have all the letters gured out, you can usually still read the message. Do y*u kno* wh*t I me*n? Did you know there is an “unbreakable” code? It relies on both sender and receiver having a copies of a “one time pad”. The one time pad has a sequence of numbers written on it. The numbers are random numbers, like the result of rolling dice, or something like that. To encode a message with a one time pad, you take your simple encoded message (A->1, B>2, ...) and add each number to the corresponding number in the one time pad, and write down the sum. To decode it, you subtract the number in the one time pad from the corresponding number in the message. It’s tedious, and it uses up a lot of numbers in the one time pad, but as long as you don’t reuse the one time pad, it can’t be broken.

7

Scytale
If you have a ribbon or thin sheet of paper with letters arranged vertically, they may hold a message encoded with a scytale. A scytale (rhymes with “Italy”) is an ancient method of easily producing and decoding coded messages. It is not a transposition cypher, because each letter in the cleartext appears in the cyphertext, but the scytale makes it di cult to read by scrambling the letters. You encode a message with a scytale by wrapping a ribbon (or in olden times, thin leather) around a sta , stick, or dowel of a speci c diameter. Once the ribbon is wrapped around, you write the message so that each letter ts on one ribbon-width. It works best if you ll the ribbon with text, because if you don’t, the empty parts of the ribbon will help “the bad guys” crack your code. To decode the message, the recipient wraps the ribbon on a sta of exactly the same diameter and reads the message o . If a “bad guy” intercepts the message, it’s very hard to gure out what it says. If you’re trying to decode a scytale, carefully go through each dowel and also experiment with which end of the ribbon to wrap rst, and in which direction. You might have to try many experiments before the message becomes clear!